The overall goal of this proposed research is to investigate developmental aspects of endocrine-cardiovascular interactions in the mammalian fetus. This includes determining: (1) if during fetal development neuroendocrine responses to non-hypotensive blood loss are mediated by signals originating from the low pressure side of the cardiovascular system, (2) if during fetal development the neuroendocrine responses to volume loss can be inhibited by neural and/or humoral signals produced by increasing pressure within the low pressure side of the cardiovascular system and finally, (3) if during fetal development the newly discovered hormone, atrial natriuretic factor -ANF, can inhibit neuroendocrine responses to volume loss. Our aim is to examine these questions at 0.75 and 0.88 gestation so that ultimately it will be possible to make comparisons of data obtained in both immature and mature fetuses studied under identical, carefully controlled circumstances. The chronically cannulated fetal lamb is the animal model chosen for study. It is possible to study the development of endocrine-cardiovascular relationships in this model in the absence of complications caused by the presence of anesthesia and surgical trauma which are inherent in experiments in acutely prepared animals. The information derived from these proposed studies will further our understanding of the ontogenesis of integration within the endocrine and cardiovascular systems and of the possible role of a new hormone (ANF) in the expression of this integration during development. Increasing our understanding of the maturation of these endocrine-cardiovascular interrelationships may prove pertinent for the improved management of the premature infant or the term infant in distress.